1. Technical Field of the Invention
The present invention pertains to apparatus for coupling radiation into the core of an optical fiber and, in particular, to apparatus for coupling pump radiation into the core of a laser oscillator or laser amplifier optical fiber.
2. Discussion of the Prior Art
In recent years, optical laser oscillators and amplifiers in an active fiber form have received increasing attention. This is because such active fiber devices combine the excellent properties of standard laser materials with the high energy confinement available in optical fibers. In particular, the round geometry of certain single-mode fibers has been adapted to fiber system applications. Such fibers exhibit large energy conversion efficiencies and excellent coupling properties to single-mode optical transmission fibers and, therefore, have important applications in fiber transmission systems and networks.
Device performance of a fiber laser device, as in any active or nonlinear waveguide, is intimately related to the efficiency with which pump radiation can be absorbed by the active material and, in particular, the active material in the fiber core. Specifically, U.S. Pat. No. 3,729,690, issued on Apr. 24, 1973, discloses a fiber configuration in FIG. 11 which is directed towards promoting efficient coupling of radiation into an active core.
The disclosed fiber configuration is described as follows at col. 17, 1. 59 through col. 18, 1. 8:
A laser component or rod of a geometry quite similar to that shown in FIG. 6 can be obtained by having a single small diameter laser fiber 130 placed eccentrically relative to its cladding glass 132 as indicated in FIG. 11. Both the geometry shown in FIG. 11 and the geometry shown in FIG. 6 are of importance in pumping arrangements in which the pumping light enters the rod through the end of the rod from substantially all directions. A principal advantage of these geometries, when used with end pumping light, is that the skew rays internally propagated down the rod are more readily intercepted by the laser element than would be the case if the active laser glass were in the center of the rod. The precise positioning of the active laser fiber 130 relative to the axis of the cladding rod 132 and their relative transverse dimensions are a matter of detail design considerations, depending upon the end results desired.
The above-described fiber configuration specifically relates to relatively short, glass fibers that were well known in the art at the time the patent was filed on Nov. 17, 1969. For that reason, the large radiation losses resulting from the radiation that scattered out of the cladding due to dirt, moisture or other inhomogeneities on the fiber surface in that fiber configuration were not important. However, that configuration is inappropriate for use with the relatively long fibers that are used today due to the large radiation losses which would result therefrom. The longer fibers used today result from the availability of low loss fibers, such as those used for telecommunications.
A further fiber configuration which is directed towards promoting efficient coupling of radiation into an active core is disclosed in U.S. Pat. No. 4,546,476 which issued on Oct. 8, 1985. The disclosed fiber configuration comprises a side-by-side arrangement of a pair of optical fibers, the first fiber providing a means for receiving pumping radiation and the second fiber being doped with an active, lasing material. The refractive indices of the first and second fibers are selected so that radiation is guided in the second, active fiber whereas pumping radiation in the first fiber is unguided. The indices of refraction were so chosen to promote transfer of pumping radiation from the first fiber to the second, active fiber. As shown in FIG. 2 of the patent, the cross-sectional area of the second active fiber 14 forms a substantial portion of the cross-sectional area of the side-by-side configuration. Specifically, the patent states at col. 3, 1. 31-38: "If the diameter of the jacket is only slightly larger than the fiber diameters, a significant portion of the pumping illumination refracted from the pumping fiber will be absorbed in the ND:YAG crystal fiber, resulting in a high energy density and thus a high inversion ratio within the ND:YAG crystal fiber to provide amplification of the optical signal which it transmits." Further, the patent discloses the use of a thin cladding as follows at col. 5, 1. 45-49: "For this reason, it will be understood that it is advantageous to maintain the envelope size of the jacket 22 as small as possible to minimize absorption by the jacket 22 and to thereby maximize absorption in the ND:YAG fiber 14." Thus, for comparable indices of refraction for the first core, second core and jacket, the number of modes of radiation which are received by the combination of first fiber and jacket, which number is proportional to the cross-sectional area of the first fiber and jacket, is approximately the same as the number of modes which could have been coupled directly into the active core because the cross-sectional areas are similiar in size. As a result, no advantage is obtained from this fiber configuration from the ratio of the area of the combined first fiber and jacket to the area of the second fiber.
In addition to the above, the patent is not directed to coupling radiation into an active, single-mode core, i.e., the patent discloses, at col. 4, 1. 50-51, that second, active fiber 14 has a core with a diameter of 100 microns. Further, an embodiment wherein the cladding or core of each of the fibers is polished to form substantially planar surfaces which are disposed adjacent to each other to promote coupling does not solve the above-identified problems.
As a result, there is a need in the art for a fiber structure containing a single-mode core which will efficiently receive a substantial portion of the radiation output by a pump source such as a laser diode, and which, after receiving that radiation, will efficiently couple that radiation into the single-mode core.